PROJECT SUMMARY/ABSTRACT Exposure of the developing brain to polychlorinated biphenyls (PCBs) and disruption of the gut microbiome have independently been implicated in the etiology of neurodevelopmental disorders (NDDs). Both phenomena likely interact by two mechanisms to cause adverse neurodevelopmental outcomes: PCB-mediated changes in the gut microbiome (1) alter the profile of neuroactive microbial metabolites distributed to the developing brain and (2) affect PCB disposition in the developing brain by modifying host and microbial PCB metabolism. A mechanistic understanding of these interactions is required to address the critical need for interventional strategies that effec- tively reduce the impact of PCB-induced NDDs on individuals, families, and society. The long-term goal is to de- termine the role of the gut microbiome?liver?brain axis in modulating susceptibility to PCB effects on the develop- ing brain, with the objective of characterizing how dose-dependent interactions between maternal exposure to an environmentally relevant PCB mixture and the gut microbiome influence neurotoxic outcomes in conventional (CV) vs. germ-free (GF) juvenile mice. We will test the central hypothesis that dysbiosis of the gut microbiome as- sociated with developmental exposure to varying doses of PCBs contributes to adverse neurodevelopmental out- comes later in life. This hypothesis is based on preliminary studies showing that: (1) PCB exposure causes dysbiosis of the gut microbiome; (2) PCBs and their metabolites are present in the rodent brain; (3) the expression of PCB-metabolizing enzymes differs between CV vs. GF mice; (4) PCB disposition differs between CV and GF mice; and (5) PCBs are developmental neurotoxicants in mice. Guided by these preliminary data, the novel hy- pothesis will be tested by (a) characterizing gut microbiome composition and function and neuroactive microbial metabolites; (b) determining how differences in host vs. microbial biotransformation affect disposition of PCBs and their metabolites; and (c) examining neuroinflammation, oxidative stress, synaptic connectivity, and behavior in dams and fetuses/pups derived from (i) CV vs.GF dams exposed to PCBs in the diet and (ii) GF dams who re- ceived a fecal transplant from PCB-exposed or vehicle control CV dams. The proposed research is innovative because it uses a systems biology approach in a state-of-the-art mouse model to assess the role of the gut mi- crobiome?liver?brain axis in modulating neurotoxic outcomes following exposure to an environmental PCB mix- ture. The anticipated outcome of these studies is a new research paradigm demonstrating that developmental exposures to PCBs mediate (1) longitudinal changes in gut microbiome composition and function and (2) alter PCB disposition in the developing brain that influence neurodevelopmental outcomes later in life. This outcome will have a significant impact on public health by informing future studies of cellular mechanisms of the devel- opmental origin of PCB neurotoxicity and, ultimately, provide critical insights regarding the plausibility of micro- biome-based approaches to diagnose and treat NDDs induced by exposure to neurotoxicants.